Issues concerning the invention will be illustrated with reference to the treatment of water to break down hydrocarbons and other organic pollutants by photocatalysis using titanium dioxide in a flow-through reactor. However, the invention can also be applied to batch and flow-through reactors for carrying out other photocatalysed chemical reactions in liquid media.
It is known to treat water, such as industrial and domestic effluent, or a fresh water supply, to remove hydrocarbons and organic compounds using titanium dioxide photocatalysis. Titanium dioxide is favoured because it is inexpensive, non-toxic, chemically stable and has a high photocatalytic activity. Titanium dioxide absorbs photons with an energy of less than or equal to its band gap energy (i.e. a wavelength of up to 388 nm in the case of anatase phase titanium dioxide) promoting a valence band electron to the conduction band. The resulting holes oxidise water to form reactive hydroxyl radicals or directly oxidise organic molecules adsorbed to the surface. The corresponding electrons reduce oxygen which is present to reactive superoxide anions. Thus, effective remediation of waste water by titanium dioxide photocatalysis requires the presence of oxygen.
One method of carrying out photocatalytic reactions is to pack photocatalyst solidly into a column through which a liquid which is to be treated flows. A water purifying unit of this type is disclosed in JP 2001-253430A (Kiyonori). However, this is not an effective use of catalyst as the transmission of light through a solidly packed column will be poor.
Another method of carrying out photocatalytic reactions is to form a layer of catalyst on the interior surface of a tube through which the liquid which is to be treated flows, or on the surface of a plate across which the liquid which is to be treated flows. In these configurations, light can be effectively transmitted to the surface of the catalyst. However, the overall effectiveness of the catalysis is limited by mass transport and the diffusion of molecules within the liquid to the catalytic surfaces.
Another method of carrying out photocatalytic reactions is to distribute fine photocatalyst particles in a reaction chamber where they float in a substantially uniform fashion. Fine photocatalyst particles will generally have a high surface area to volume ratio which would be expected to aid photocatalysis. However, light transmission can be poor as such particles will have a high cross-section to volume ratio. If the catalyst is sufficiently fine to be distributed in a substantially uniform fashion throughout the reaction chamber, it will be difficult to separate. When used in a batch mode, the catalyst may take weeks to settle once the reaction has been completed. When used in a flow-through mode, it can be very difficult to filter the catalyst from the outflow and the particles will flow, with the liquid which is to be treated, into the outlet filters.
International patent application publication number WO 2005/033016 (Robertson et al.) discloses apparatus for the remediation of water using titanium dioxide photocatalysis including a container within which the photocatalysed reaction takes place and means to move the container. The use of titanium dioxide pellets with a mean grain size of 0.1 to 50 mm is disclosed. Thus, the apparatus disclosed in this application enables the effective use of mobile photocatalyst particles of a sufficiently large size to enable them to be conveniently separated from the liquid which is to be treated, without mass transport limitations. This apparatus is effective, but the use of moving parts could limit its usefulness in some applications.
It would be desirable to provide a photocatalytic reactor which uses mobile photocatalyst particles of a sufficiently large size to enable them to be conveniently separated from the liquid which is to be treated, and which minimizes mass transport limitations, but does not require moving parts, or minimizes the use of moving parts.